1. Field of the Invention
The present invention relates to an electromagnetic flowmeter and an electromagnetic flow-rate measuring method for measuring the rate of flow of a conductive fluid, and more particularly to an electromagnetic flowmeter for minimizing measurement errors due to induced noise without making zero-point adjustments that involve stopping the flow of a fluid the rate of flow of which is to be measured to thereby provide a significant improvement in measurement precision and a method therefor.
2. Description of the Related Art
In general, an electromagnetic flowmeter is arranged to apply an electromagnetic field in a direction orthogonal to a conductive fluid flowing through a measurement tube, detect electromotive force electromagnetically induced in the fluid by using two electrodes inside the tube, and convert it to a value corresponding to the rate of flow of the fluid. In recent electromagnetic flowmeters, a low-frequency exciting type of electromagnetic flowmeter is widely used, which is also called a square-wave exciting type and which is excellent in zero point stability in comparison with an alternating-current, i.e., sine wave, exciting type and a direct-current, i.e., constant current, exciting type.
In a low-frequency exciting type of electromagnetic flowmeter, an exciting current flowing through an exciting coil is switched between two fixed values periodically, i.e., the polarity of the excitation current is changed periodically, and electromotive force generated between the two electrodes inside the tube is amplified and sampled when the exciting current becomes each of the fixed values once. By producing the difference between successive sampled values; a flow rate value can be obtained which is free of two kinds of noise: a DC noise caused by an offset voltage of an amplifier or an electrochemical phenomenon on surfaces of the electrodes, and other is an electromagnetically induced noise induced by the link between an exciting coil and a loop which is composed of the electrodes, fluid and the amplifier.
However, a problem with such a low-frequency exciting type of electromagnetic flowmeter is that the zero point drifts to produce an error in flow rate measurement unless electromotive force between the electrodes is sampled after a lapse of a sufficient length of time since the exciting current has reached a fixed value. Making the switching period of the exciting current so long as to stabilize the zero point would prolong the time interval for obtaining the flow rate signal, resulting in poor responsiveness.
To stabilize the zero point, such a novel electromagnetic flowmeter as disclosed in Japanese Unexamined Patent Publication No. 2-16852 was devised. FIG. 10 is a block diagram of this electromagnetic flowmeter, and FIG. 11 is a timing diagram illustrating its operation. The flowmeter comprises an excitation circuit 1 for supplying an excitation current, an electromagnetic flowmeter transmitter 2 responsive to the exciting current from the excitation circuit 1 to produce electromotive force e.sub.a proportional to the rate of flow of the fluid, and a signal processing circuit 3 for obtaining a flow rate value corresponding to the electromotive force e.sub.a.
In the excitation circuit 1, a constant current source 4 produces a constant current and switches 5a and 5b are respectively responsive to pulses P1a and P1b shown at (a) and (b) in FIG. 11 to vary the polarity of the constant current from the constant current source 4 in the order zero, negative, and positive and supply the constant current of varying polarity to an excitation coil 6 in the electromagnetic flowmeter transmitter 2.
In the electromagnetic flowmeter transmitter 2, the exciting coil 6 is responsive to application of an exciting current Iw (shown at [c]in FIG. 11) from the exciting circuit 1 to produce a magnetic field that is applied to a measurement tube 7 in a direction orthogonal to its axis. Two electrodes 8a and 8b disposed on the inner wall of the tube apply electromotive force e.sub.a induced in the fluid by that magnetic field as shown at (d) in FIG. 11 to an AC amplifier 9 in the signal processing circuit 3.
In the signal processing circuit 3, the AC amplifier 9 amplifies the electromotive force e.sub.a detected by the electrodes 8a and 8b, and a switch 10 samples the amplified output twice during the time interval when the excitation current is zero and once during the time interval when the exciting current is positive or negative in accordance with the timing of sampling pulses P2 shown at (e) in FIG. 12. Each of the resulting samples is applied to an analog-to-digital (A/D) converter 11 in sequence.
The A/D converter 11 converts each of the samples to a digital form and applies it to a microprocessor 12.
The microprocessor 12 performs calculations on these digital samples to remove an offset voltage component Vf and a noise component Vn from the electromotive force, thereby obtaining signal components Vs1 and Vs2 proportional to only the flow rate of fluid. The signal components Vs1 and Vs2 are applied to a digital-to-analog (D/A) converter 13. The microprocessor 12 has functions of providing the pulses P1a and P1b to the switches 5a and 5b, the pulses P2 to the sampling switch 10, and control pulses P3 to a sample and hold circuit 14 following the D/A converter 13.
The D/A converter 13 converts the results of calculations by the microprocessor 12 to analog signals which are in turn applied to the sample and hold circuit 14. The sample and hold circuit 14 samples and holds the analog signals at the times of the pulses P3 shown at (f) in FIG. 11 to produce output voltages e.sub.0.
Thus, this type of electromagnetic flowmeter compensates for the difference between a noise component when the exciting current is positive or negative and a noise component when the excitation current is zero to effectively remove noise components which cause the zero drift, thereby stabilizing the zero point while preventing the responsibility from lowering without making the switching interval long.
In the electromagnetic flowmeter, however, one sampling time has to be very short because sampling is performed twice during a time interval when the excitation current is zero and once during a time interval when the excitation current is positive or negative. Thus, the measurement is subject to the influence of bubbles or solid material in the fluid during sampling, which will decrease the reliability of measurement.
On the other hand, making the sampling time for electromotive force e.sub.a produced between the electrodes so long as to circumvent the influence of such bubbles and solid material would require a long excitation period or cycle which results in poor responsiveness. Moreover, slurry fluids, such as pulp liquid, have 1/F characteristics in which the lower the noise frequency, the higher the noise level becomes. Thus, the signal-to-noise ratio deteriorates as the excitation cycle increases, that is, the excitation frequency decreases.